Optical disk apparatus and optical disk processing method

ABSTRACT

An optical disk apparatus has a first optical system having an objective lens with a first NA, a second optical system having an objective lens with a second NA lower than the first NA, an addition mechanism for adding focus error signals detected by the first and second optical systems, and an alignment mechanism for aligning the objective lens in accordance with the addition result of the addition mechanism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.09/916,444, filed Jul. 30, 2001, now U.S. Pat. No. 6,829,203 the entirecontents of which are incorporated herein by reference. This applicationis based upon and claims the benefit of priority from the prior JapanesePatent Application No. 2001-126115, filed Apr. 24, 2001, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disk apparatus for recordinginformation on or reproducing information from an information recordingtrack of an optical disk by irradiating the optical disk with a lightbeam. More particularly, the present invention relates to focus errordetection of a light beam in this optical disk apparatus. Also, thepresent invention relates to an optical disk processing method.

In recent years, various studies and developments have been made for thepurpose of improving the recording density of an optical disk. In anoptical disk apparatus, an attempt is made to reduce the spot size of anoptical spot by increasing the optical NA (Numerical Aperture) oradopting a short-wavelength laser. When the optical NA is increased,problems of stricter specifications of the tilt of an objective lens anda short distance between the objective lens and optical disk surface arepointed out.

As is well known, the specifications of the tilt of the objective lenscan be relaxed by decreasing the distance from the optical disk surface(light entrance surface) to the recording surface, i.e., the thicknessof a substrate. A sample optical disk having a shorter distance than theexisting DVD or the like (which has a substrate having a thickness ofabout 0.6 mm), e.g., adopting a substrate having a thickness of about0.1 mm has been prepared.

On the other hand, a high NA also leads to a decrease in distancebetween the objective lens and information recording surface. Thedistance between the objective lens and optical disk in the existing DVDis 1 mm or more. However, when an NA higher than 0.8 is adopted, theobjective and optical disk adjoin at a distance less than 0.2 mm. Whenthe objective lens and optical disk adjoin via such a short distance,the most serious problem is a collision of the objective lens againstthe optical disk. Such collision takes place when focus control runsaway upon a focus lead-in operation or upon mixing of any disturbance inlight reflected by the optical disk due to the influence of scratches,fingerprints, and the like on the optical disk. Hence, a stable focuslead-in operation and a focus control system which has high resistanceagainst the influence of scratches, fingerprints, and the like aredemanded.

As a conventional method of avoiding such collision, a method of formingan arcuated pattern on the objective lens on the optical disk surfaceside is proposed (Jpn. Pat. Appln. KOKAI Publication No. 2000-20985).The objective lens with such a structure exploits the generation of afloating force with respect to the optical disk due to the air flowgenerated in the gap between the optical disk and objective lens uponrotation of the optical disk. The objective lens is passively aligned toa position where the floating force and the driving direction in thefocus direction balance. However, in this method, since the floatingforce of the objective lens with respect to the optical disk changesdepending on the rotational speed of the disk, the floating amountreadily changes due to the influences of disk rotation variations. Whenthe rotational speed of the disk is not constant, the objective lensbecomes unstable in this control method.

In order to attain adequate focus control, servo control of aconventional optical disk apparatus, which uses a focus error signal ofa focus system is preferable. For example, Jpn. Pat. Appln. KOKAIPublication No. 2000-011401 proposes a method of realizing stable focuslead-in operation by adding signals from focus error detection systemshaving two different focus detection ranges. According to this method, astable focus lead-in operation is realized by adding a focus errorsignal obtained by a detection optical system that can assure a broadfocus error detection range, and a focus error signal obtained by adetection system which has a narrow focus error detection range but canassure high detection sensitivity. At the same time, precise focusalignment is realized. However, when scratches, dust, fingerprints, andthe like become attached to the disk surface, the amplitude of the focuserror signal itself used as a servo signal becomes small, the signal issusceptible to disturbances, and so forth. Furthermore, when the NA ishigh, since the spot size on the disk surface becomes small, the signalis readily influenced by disturbances, and the focus servo consequentlyruns away. Such an unstable state similarly occurs even in a focuslead-in operation, thus disturbing a stable focus lead-in operation.That is, since two different focus errors are generated using returnlight from a single optical spot formed on the information recordingsurface of the optical disk, an unstable state of the optical spotformed by a high-NA optical system cannot be avoided. As a result, theconventional servo control is vulnerable to disturbances that depend onthe surface state of the optical disk such as scratches, dust,fingerprints, and the like, and the focus servo readily runs away due tothe influences of disturbances.

Once the servo runs away, the objective lens may collide against thedisk to damage not only the optical disk but also the objective lens,and information recording/reproduction may be disabled. In this way, thefactors necessary for realizing an optical disk apparatus compatiblewith high-density optical disks are to realize a focus error detectionsystem with which a focus servo hardly runs away even under theinfluence of scratches, dust, fingerprints, and the like, and a focuserror detection system which hardly collides against the disk, and torealize a stable focus lead-in operation at the same time.

As described above, when the conventional focus error detection circuitis applied to a high-NA optical system, the objective lens is highlylikely to collide against the disk. Also, the focus servo itselfinevitably runs away due to the influence of scratches, dust, andfingerprints attached to the disk. When the focus servo runs away, sincethe distance between the objective lens and disk is small, the objectivelens collides against the optical disk.

Furthermore, in the case of a high-density optical disk having atwo-layered information recording surface, a focus error detection rangeis inevitably narrowed down. For this reason, focus control performancedeteriorates. When focus control fails, the objective lens inevitablycollides against the optical disk. For this reason, it is indispensableto realize a stable focus lead-in operation.

Moreover, in an optical disk having a thin substrate, since individualoptical disks have different substrate thicknesses, substrate thicknessvariation correction such as spherical aberration correction or the likemust be performed. Hence, a control system which executes focus controlwhile making spherical aberration correction that influences focuscontrol must be realized.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and has as its object to provide the following optical diskapparatus and optical disk processing method:

(1) an optical disk apparatus which can realize stable focus control andfocus lead-in operation even when a high-NA optical system is used; and

(2) an optical disk processing method which can realize stable focuscontrol and focus lead-in operation even when a high-NA optical systemis used.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a perspective view showing the structure around an objectivelens actuator of an optical disk apparatus according to the firstembodiment of the present invention;

FIG. 2 is an enlarged view of the structure around the objective lensactuator of the optical disk apparatus according to the first embodimentof the present invention;

FIG. 3 is a schematic block diagram showing the arrangement of principalparts associated with focus control of the optical disk apparatusaccording to the first embodiment of the present invention;

FIG. 4 is a chart for explaining signals upon focus lead-in control ofthe optical disk apparatus according to the first embodiment of thepresent invention;

FIG. 5 is a schematic block diagram (part 1) showing the arrangement ofprincipal parts of a fine adjustment focus system of the optical diskapparatus according to the first embodiment of the present invention;

FIG. 6 is a flow chart showing the reproduction and recording processesusing objective lenses having different NAs of the optical diskapparatus according to the first embodiment of the present invention;

FIG. 7 is a schematic block diagram (part 2) showing the arrangement ofprincipal parts of the fine adjustment focus system of the optical diskapparatus according to the first embodiment of the present invention;

FIG. 8 is a schematic block diagram showing the arrangement of principalparts of a focus error detection system of an optical disk apparatusaccording to the second embodiment of the present invention;

FIG. 9 is a view showing the structure of an objective lens of a high-NAoptical system of the optical disk apparatus according to the secondembodiment of the present invention;

FIG. 10 is a view showing signals of focus error detection systems ofthe optical disk apparatus according to the second embodiment of thepresent invention;

FIG. 11 is a graph showing an example of a computed focus error signal;

FIG. 12 is a chart showing an example of a computed focus error signal;

FIG. 13 is a view for explaining a focus error detection method;

FIG. 14 is a view for explaining the storage operation for synchronouslystoring rotation angle information from a rotation angle detectioncircuit of a spindle motor, and the control output of a first focuscontrol circuit;

FIG. 15 is a view for explaining a case wherein a second actuator doesnot retract upon recording/reproduction by a first optical system;

FIG. 16 is a view for explaining a case wherein the second actuator hasretracted upon recording/reproduction by the first optical system;

FIG. 17 is a view showing the recording/reproduction state by a secondoptical system;

FIG. 18 is a view showing a state wherein the second actuator ismechanically restrained;

FIG. 19 is an enlarged view showing the state wherein the secondactuator is mechanically restrained; and

FIG. 20 is a view showing a state wherein the second actuator iselectromagnetically restrained.

DETAILED DESCRIPTION OF THE INVENTION

The first embodiment of the present invention will be described belowwith reference to the accompanying drawings.

First Embodiment

Two objective lens actuators are provided, and focus lead-in control isperformed by learning the surface run-out.

FIG. 1 is a perspective view showing the structure of an optical diskapparatus having a focus error detection circuit according to thepresent invention. FIG. 2 is an enlarged view of the structure around anobjective lens actuator. This structure has two different opticalsystems, and an optical head has two objective lenses and actuators, sothat information can be reproduced from or recorded on a predeterminedoptical disk using a low-NA optical system, and information can bereproduced from or recorded on another predetermined optical disk usinga high-NA optical system.

An optical disk 1 having an information recording surface is attached toa spindle motor 2, and its rotation is controlled. An optical head forreproducing/recording information by forming an optical spot on theinformation recording surface of the optical disk 1 is supported to bemovable in the radial direction of the optical disk, and is aligned inthe radial direction by a feed motor such as a stepping motor 3 or thelike. Two objective lens actuators 5 and 6 are mounted on a carriage 4on which the optical head is mounted, and precisely align an opticalspot with respect to an information recording track formed on theinformation recording surface in two directions, i.e., a focus directionas a direction perpendicular to the disk, and a track direction as adirection crossing the track. Of the two objective lens actuators, thefirst objective lens actuator 5 is provided for a low-NA optical system,and forms an optical spot on the information recording surface of anoptical disk via a first objective lens 7 upon receiving a laser beamhaving a wavelength ranging from, e.g., 650 nm to 780 nm. The firstobjective lens actuator 5 is supported by a first objective lens supportwire 15. On the other hand, the second objective lens actuator 6 isprovided for a high-NA optical system, and forms an optical spot on theinformation recording surface of an optical disk via a second objectivelens 8 upon receiving a laser beam having, e.g., a wavelength of around400 nm. The second objective lens actuator 6 is supported by a secondobjective lens support wire 16. Note that an optical disk used uponforming an optical spot via the first objective lens, and an opticaldisk used upon forming an optical spot via the second objective lenshave different physical properties. That is, an optical disk from/onwhich information is recorded/reproduced using a first optical systemallows information recording/reproduction using a laser beam having awavelength ranging from 650 nm to 780 nm, and is used for reproductionof a CD or DVD or recording/reproduction of a DVD-RAM or DVD-R orDVD-RW. On the other hand, an optical disk from/on which information isrecorded/reproduced using a second optical system allows informationrecording/reproduction using a laser beam having a wavelength of around400 nm, and has a surface recording density higher than a DVD or thelike. In addition, the size of the optical spot formed on theinformation recording surface in this case is smaller than that formedby the first optical system. The first and second optical systems areselectively used to reproduce/record information from/on these two ormore different optical disks using a plurality of laser beams havingdifferent wavelengths.

The focus lead-in method and focus control method in the optical diskapparatus with this structure and, more particularly, the focus controlmethod for an optical disk compatible with the second optical systemwith high NA will be described below with reference to the block diagramin FIG. 3 that shows principal parts for focus control, and the flowchart in FIG. 6.

Disk discrimination is done first (ST3). That is, it is checked if theoptical disk 1 attached to the spindle motor 2 is an optical disk thatallows information recording/reproduction using the first optical systemor an optical disk that allows information recording/reproduction usingthe second optical system. The disk discrimination starts with focuscontrol by the first optical system that uses the low-NA first objectivelens (ST2). Light reflected by the optical disk enters a first PD (PhotoDetector) 21 via a beam splitter 11 and focusing lens 13. Alternatively,that reflected light enters a second PD 26 via a beam splitter 12 andfocusing lens 14. That is, sequentially voltage varying a kick pulse forfocus lead-in is input to the first objective lens actuator while usinga focus error signal detected by the first PD 21 for focus errordetection arranged in the first optical system. FIG. 13 shows anastigmatism detection system as a general focus error signal generationsystem. In FIG. 13, a value given by (A+C)−(B+D) of a 4-split PD is afocus error amount. At this time, the second objective lens actuator 6is preferably displaced and restrained in a direction away from theoptical disk to avoid collision against the target optical disk surface(ST1). In the aforementioned operation sequence, the first objectivelens actuator 5 undergoes focus control with respect to the optical disk1 by a first focus control circuit 23 using a focus error signaldetected by a low-NA first focus error detection circuit 22. Thisoperation is executed in correspondence with the number of laser lightsources of the first optical system in descending order of wavelengths.For example, focus control is executed using an incoming laser beam of780 nm, and the next focus control is executed using an incoming laserbeam of 650 nm. After focus control attempts using laser beams of allthe wavelengths of the first optical system are made, if an informationsignal can be reproduced, or a reproduction signal is a desired signal,it is determined that the target optical disk is the one compatible withthe first optical system (ST3). In this case, an informationrecording/reproduction process using the first optical system isexecuted (ST4).

After focus control attempts using laser beams of all the wavelengths ofthe first optical system are made, if an information signal cannot bereproduced or a reproduction signal is not a desired signal, it isdetermined that the target optical disk is not an optical diskcompatible with the first optical system but an optical disk compatiblewith the second optical system (ST3), and an optical system switchoperation is executed.

Prior to the switch operation, surface run-out of the target opticaldisk is learned using a control signal upon focus control of the firstobjective lens actuator 5 (ST5). This control signal can use a laserbeam of any wavelength of the first optical system, but preferably usesa laser beam of the longest wavelength, e.g., a laser beam of awavelength of, e.g., 780 nm. In the learning operation, a control signalof the actuator, which is output in accordance with surface run-out, isstored by a surface run-out phase estimation circuit in synchronism withthe rotation angle information of the spindle motor. The first andsecond objective lens actuators generally undergo electromagnetic drivecontrol. The control signal of such an electromagnetically drivenactuator is supplied as an acceleration signal of the actuator. When theactuator is driven to track surface run-out as a disturbance factor of alow frequency such as a disk rotation frequency, the drive signal can beprocessed as a signal which changes nearly in phase with the positionsignal. Especially, the focus lead-in operation is performed on the diskinner peripheral portion where an expected surface run-out disturbanceis small, i.e., a region where the rotation frequency for rotating thedisk is relatively high (around 40 Hz). It is not basically difficult toestimate the surface run-out phase in this frequency domain. Forexample, the deviation of the actuator is estimated from the drivesignal based on the known dynamic characteristic response model of theactuator, and the surface run-out phase is estimated since it isequivalent to the surface run-out deviation.

A surface run-out phase estimation circuit 24 stores the rotation angleinformation from a spindle motor rotation angle detection circuit 25,and the control output of the first focus control circuit 23 insynchronism with each other. This storage operation will be explainedbelow with reference to FIG. 14. The rotation angle detection circuit 25can detect rotation angle information, and every angle obtained bydividing 360° into a plurality of angular ranges. For example, when thecircuit 25 detects rotation angle information every 60°, the focuscontrol output is sampled in synchronism with the rotation angleinformation every 60° and, for example, a voltage output value is storedas a sample value shown in FIG. 14. The stored voltage value isinformation which allows a rough estimation of the period and phasevalues of the drive signal by linear interpolation. After the surfacerun-out state is stored in synchronism with the rotation angle, theoptical system is switched from the first optical system to the secondoptical system while controlling the rotation of the optical disk at anidentical rotational speed. This switch operation is implemented bysupplying a laser beam of a predetermined wavelength to the secondobjective lens actuator 6, and inputting a kick pulse control signal forfocus lead-in to the second objective lens actuator 6 via a second focuscontrol circuit 28 (ST6). This lead-in pulse is a drive signal formoving the second objective lens 8 toward the optical disk. Therefore,the objective lens actuator 6 moves toward the optical disk surfacesimultaneously with input of the kick pulse control signal. At the sametime, the objective lens actuator 6 is controlled by the second focuscontrol circuit 28 using a focus error signal which is detected by thesecond PD 26 provided in the second optical system and is computed by asecond focus error detection circuit 27. When the objective lens 8 movesto a position where the second focus error detection circuit 27 candetect a focus error signal and the relative velocity between theobjective lens 8 and the information recording surface of the opticaldisk 1 is smaller than a predetermined velocity, the focus lead-inoperation succeeds (ST7). The surface run-out velocity is estimated fromthe surface run-out state estimated by the surface run-out phaseestimation circuit 24, so that the relative velocity falls within thepredetermined range, and lead-in control succeeds. Furthermore, alead-in pulse generation circuit 29 superposes a kick pulse and brakepulse on the focus control signal at an optimal timing at which avelocity relative to the moving velocity of the objective lens becomessmall. More specifically, such timing is preferably set near a regionwhere the surface run-out amount is closest to the objective lens 8,since the surface run-out velocity of the optical disk is low, and therelative velocity with respect to the objective lens can be reduced. Asdescribed above, the lead-in operation is performed using the focuserror signal of the second focus error detection circuit 27 provided inthe second optical system. The relationship between the signal of thesurface run-out phase estimation circuit, lead-in pulse, and focus errorsignal is as summarized in FIG. 4.

After the focus lead-in operation is performed in synchronism with thesurface run-out phase, the second objective lens actuator 6 undergoesfocus control with respect to the information recording surface of theoptical disk. However, since the NA is high, the end face of theobjective lens is aligned to a position 0.2 mm or less from the opticaldisk surface. After it is confirmed that an optical spot formed by thesecond objective lens 8 is in focus on the information recording surfaceof the optical disk, a fine adjustment operation of a sphericalaberration correction system provided in the second optical systemstarts (ST8). The fine adjustment operation will be explained belowusing FIG. 5 as a block diagram that shows principal parts of a focusfine adjustment system.

A spherical aberration correction system 30 shown in FIG. 5 comprises,e.g., relay lenses and the like, and adjusts the laser beam size or thespread angle of the laser beam that enters the objective lens bycontrolling, e.g., the distance between two lenses. That is, this systemconsequently corrects any spherical aberration of an optical spot. Inthe high-NA second optical system, the thickness from the optical disksurface to the information recording surface is around 0.1 mm, andindividual optical disks suffer thickness nonuniformity. To correct thethickness nonuniformity, such a spherical aberration correction systemis often adopted. The spherical aberration correction system 30 providedfor this purpose is preferably adjusted by a spherical aberrationcorrection control circuit 32 to maximize the return light amount, whilea second return light detection circuit 31 detects the sum total ofreturn light amounts from the optical disk detected by the second PD 26in a state wherein the second objective lens actuator undergoes focuscontrol. This process is performed in consideration of the fact that thereturn light increases when the spherical aberration of the optical spotformed on the information recording surface is corrected. With thisarrangement, a high-quality optical spot can be obtained.

In this way, the focus lead-in operation of the second optical system iscompleted. Upon completion of the focus lead-in operation, predeterminedinformation recorded on the optical disk is read and reproduced toconfirm if the target optical disk is compatible with the high-NAoptical system, and a desired information signal is recorded orreproduced (ST9).

Note that surface run-out learning in the aforementioned operation maybe performed using another optical system in place of the first opticalsystem. In the surface run-out learning step (ST5) and the lead-in step(ST6) at the optimal timing, the first optical system need not beswitched to the second optical system, and the focus lead-in operationof the second objective lens actuator may be realized while the firstobjective lens actuator of the first optical system undergoes focusservo control to information recording surface run-out. In such a case,the disk need not be controlled at the same rotational speed as thatupon surface run-out learning, and may be rotated at an arbitraryrotational speed.

In the lead-in sequence, the lead-in pulse may be a gradually varyingsignal. Then the lead-in signal may control the objective lens actuatorfirstly to retract away from the surface of the optical disk, andgradually to approach onto the surface of the optical disk.

In the first optical system, a plurality of laser beams enter a singleobjective lens. Alternatively, the first optical system may comprise,e.g., a single objective lens actuator having two objective lenses, twolaser light sources, and a focus error detection circuit correspondingto the two laser light sources. More specifically, the first opticalsystem may comprise a first objective lens actuator that holds anobjective lens corresponding to a wavelength of 780 nm, and an objectivelens corresponding to a wavelength of 650 nm, laser beams of the twowavelengths, and a detection system.

While the first optical system performs surface run-out learning, thesecond objective lens actuator is preferably restrained at a positionaway from the optical disk surface. The positional relationship betweenthe first and second objective lens actuators will be explained belowwith reference to FIGS. 15 to 20. If the second objective lens actuatoris not retracted and restrained while the focus servo is activated bythe low-NA first objective lens actuator, the positional relationshipbetween the first and second objective lens actuators uponrecording/reproduction using the first optical system is as shown inFIG. 15. That is, the first objective lens actuator is driven to be ingood focus with an information recording surface 71 of the optical disk.At this time, if a surface 70 of the optical disk suffers surfacerun-out, it may collide against the second objective lens. To avoid thecollision, the second objective lens actuator is retracted and isrestrained at that position, as shown in FIG. 16. Uponrecording/reproduction using the second optical system, the positionalrelationship between the first and second objective lens actuators is asshown in FIG. 17. The restraint described in FIG. 16 may be attained byproviding a mechanical projection 73 to an objective lens holdingmember, and by restraining the projection 73 by a rotary member 75attached to the distal end of a motor 74, as shown in FIGS. 18 and 19.Also, an electromagnetic coil 76 provided in the second objective lensactuator may be electromagnetically restrained by a projection 77 of thecarriage 4, as shown in FIG. 20. Under such restraint, the lead-inoperation may be realized by the resilience of a support spring uponreleasing from the restraint without using any drive control signalbased on a lead-in pulse. For example, when the objective lens actuatoris held by electromagnetic restraint in a direction farther away fromthe optical disk surface than a mechanical neutral position, theobjective lens actuator 6 moves by the resilience toward the opticaldisk surface simultaneously with release. The lead-in operation may beattained by this moving velocity.

In the above arrangement, the focus lead-in operation is not performedat an in-focus point with respect to the optical disk surface but isperformed with respect to only the information recording surface. Such alead-in operation is realized by, e.g., a counter so as to lead theobjective lens to the second change position of an error signal whileobserving a focus error signal. However, since the operation of such acounter is often unstable, focus control may be executed for the opticaldisk surface using a focus error signal observed with respect to theoptical disk surface, and the focus lead-in operation may be performedwith respect to the information recording layer by focus jump. Likewise,in the case of an optical disk having a plurality of informationrecording layers, after the focus lead-in operation is made for thefront information recording layer, focus jump is preferably made in turnto the next layer. The focus lead-in operation may be performed byselecting a target information recording layer by the counter operation.However, in consideration of stability of the counter operation, areliable focus lead-in operation can be achieved by executing thelead-in operation with respect to the front layer which allows thelead-in operation.

In a focus system fine adjustment using the spherical aberrationcorrection system, the spherical aberration correction system 30 may beadjusted to reduce a DC component while observing the DC component ofthe focus control signal of the second focus control circuit 23, asshown in FIG. 7. At the same time, the return light amount may bedetected to correct any spherical aberrations to an optimal position.

The second embodiment of the present invention will be described belowwith reference to the accompanying drawings.

Second Embodiment

A focus error detection optical system generates and mixes two focuserrors.

FIG. 8 is a block diagram showing the arrangement of an optical diskapparatus having a focus error detection circuit of the presentinvention. Since the arrangement of the low-NA first optical system andthe like is the same as that of the first embodiment, FIG. 8 shows thearrangement of only the high-NA second optical system.

In this arrangement, a phase converter 51 having a function of shiftingabout ¼λ (wavelength) only the transfer phase around the optical axiscenter is inserted in the optical path of the high-NA optical system.Some light components of light that enter the second objective lens 8,especially, light components near the optical axis center are notfocused by a first component lens 62 that forms the second objectivelens, but are focused by only a second component lens 61, forming a spoton the information recording surface in the optical disk. FIG. 9 showsthe structure of such a second objective lens 8. Light components nearthe optical axis center, which are focused by only the second componentlens 61, form an optical spot with a large spot size on the informationrecording surface due to the low NA. Also, light beam componentsslightly outside the optical axis center, which do not undergowavelength shift by the phase converter, are focused with the high NA bythe first and second component lenses, thus forming a spot on theinformation recording surface in the optical disk. This spot size issmall since the NA is high. The respective incoming light components aresplit by a polarization beam splitter 52 after they are reflected by theinformation recording surface. The light beam components around theoptical axis center enter a third PD 64 via a focusing lens 53, and aredetected as a third focus error signal by a third focus error detectioncircuit 55. On the other hand, the light beam components slightly awayfrom the optical axis center enter the second PD 26 via the beamsplitter 12 and the focusing lens 14. A second focus error detectioncircuit 56 detects a second focus error signal on the basis of the lightintensity detected by the PD 26. The second and third focus errorsignals are input to a second focus error arithmetic circuit 57 andundergo addition/subtraction. The calculated focus error signal is usedto drive the second objective lens actuator 6 by the second focuscontrol circuit.

The arithmetic operation made by the second focus error arithmeticcircuit 57 at that time will be described below with reference to FIG.10. The second focus error detection circuit 56 detects a focus errorsignal with respect to the information recording surface of the opticaldisk using a small optical spot formed with high NA. For this reason,the deviation range having the sensitivity of the focus error signal isnarrow, and the spot size on the optical disk surface decreases. On theother hand, the third focus error detection circuit 55 detects a focuserror signal with respect to the information recording surface of theoptical disk using a relatively large optical spot formed with low NA.For this reason, a relatively broad deviation range having thesensitivity of the focus error signal can be assured, and the spot sizeon the optical disk surface is large. These focus error signals havingtwo different properties are, e.g., added to obtain a focus error signalfor driving the second objective lens actuator 6.

The focus error signal calculated, as shown in FIG. 11, has propertiesof broadness in detection range of the third focus error signal androbustness against the influences of scratches and dust on the opticaldisk surface, and the second focus error signal has a high detectionresolution that can assure accurate focus alignment. Let FE2 be thesecond focus error signal, and FE3 be the third focus error signal.Then, a calculated focus error signal FE-cal is given by:FE−Cal=FE2+α×FE3where α is an arbitrary positive value. Since the focus error signal iscalculated in this way, the focus lead-in operation can be stablyperformed using the properties of the third focus error signal, andfocus alignment can be accurately performed owing to the properties ofthe second focus error signal. Even when the second focus error signalis disturbed by the influence of scratches, dust, and the like on theoptical disk surface, the third focus error signal is robust against thedisturbances, and can be detected. For this reason, although thealignment precision of focus control slightly deteriorates, a moderatefocus control that tracks surface run-out of the optical disk can beimplemented while preventing the objective lens from colliding againstthe optical disk due to being out of control.

Note that the third focus error detection circuit 55 with the abovearrangement can slightly electrically offset the third focus errorsignal and can detect an in-focus point on the objective lens side infront of the information recording surface of the optical disk. In thiscase, the focus error signal calculated by the second focus errorarithmetic circuit 57 can be formed to have a plurality of focal pointpositions, as shown in FIG. 12. In this case, after the focus lead-inoperation is made with respect to the front in-focus position, focusjump is made to the back in-focus position as that of the second focuserror detection system, thus achieving accurate focus alignment to thetarget information recording surface. At the same time, with thisarrangement, even when the focus error signal of the second focus errordetection system cannot be detected due to the state of the optical disksurface, the objective lens moderately moves away from the optical disksurface and undergoes focus control with respect to the front in-focusposition, thus avoiding collision of the objective lens.

The aforementioned arrangement is also effective when focus control ismade for an optical disk having a plurality of information recordinglayers. When an optical disk which allows informationrecording/reproduction using the high-NA optical system has a pluralityof information recording layers, the interlayer distance betweenneighboring information recording layers must be small. Morespecifically, the distance is preferably 30 μm to 50 μm. This is becausea reflection film of the back layer is formed by a metal film such asaluminum, and a stable focus error signal is expected even when the NAis low. However, with this arrangement, since focus error detection of afirst information recording layer, which is formed in front of theobjective lens, and that for a second information recording layer, whichis formed behind the objective lens must be processed not to interferewith each other, the respective focus error detection ranges become verynarrow. More specifically, an error detection system within the range ofabout ±3 μm is formed. In such case, the second focus error arithmeticcircuit can calculate a focus error signal that can implement stablefocus control, on the basis of the third focus error signal detected bythe third focus error detection circuit and the second focus errorsignal detected by the second focus error detection circuit. At thistime, the third focus error detection circuit preferably detects thefocus error signal using return light especially from the backinformation recording layer.

In the above arrangement, since the focus control lead-in operation,fine adjustment operation, and jump operation are the same as those inthe first embodiment, a description thereof will be omitted.

In the above arrangement, since the focus lead-in control can be stablyperformed due to the properties of the third focus error signal, surfacerun-out learning using the low-NA first optical system need not beperformed.

As described above, according to the present invention, the followingeffects can be obtained.

When low- and high-NA optical systems are present and respectively havedifferent objective lens actuators, a first objective lens actuator thatholds a first objective lens of the low-NA optical system executes focuscontrol first to learn the surface run-out amount of the target opticaldisk. After that, if it is determined that the target optical disk iscompatible with information recording/reproduction using the high-NAoptical system, a second objective lens actuator that holds a secondobjective lens of the high-NA optical system executes the focus lead-inoperation at an optimal timing with reference to the learned surfacerun-out phase, thus stably switching the focus control.

Even in the high-NA optical system, low-NA focus error detection is madeusing light components around the optical axis center, and a signalobtained by adding two focus error signals obtained by high- and low-NAfocus error detection systems is used as a focus error signal for focusalignment of an objective lens. With this arrangement, an in-focus pointof the low-NA focus error detection system is formed slightly on thedisk surface side compared to that of the high-NA focus error detectionsystem, and a drive force that becomes a large repulsion force can begenerated upon defocusing the objective lens in a direction to collideagainst the disk. Furthermore, with this arrangement, since the low-NAfocus error detection system can form a larger spot on the disk surfacethan the high-NA focus error detection system, a focus error signalrobust against the influence of dust or scratches on the disk surfacecan be obtained. For this reason, even when a signal from the high-NAfocus error detection system decreases due to the influence of scratchesand dust, the objective lens can be prevented from colliding against thedisk due to runaway of the focus servo, and servo control is made in adirection away from the disk.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit and scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical disk apparatus comprising: a phase converter forconverting a transfer phase of a light beam component of a light beamirradiated from a light source, the light beam including a first lightbeam component being away from an optical axis center in the directionperpendicular to the optical axis by a predetermined range and a secondlight beam component being within the predetermined range from theoptical axis center in the direction perpendicular to the optical axis,the phase converter converting the transfer phase of the second lightbeam component; an objective lens including a first condensing lens anda second condensing lens, the first condensing lens condensing the lightbeam including the second light beam component phase-converted by thephase converter, the second condensing lens condensing the light beamcondensed by the first condensing lens; an objective lens holder whichholds said objective lens and is supported to be drivable in an opticalaxis direction of said objective lens and in one direction perpendicularto the optical axis; a focusing actuator for driving said objective lensholder in the optical axis direction; first focus detection means fordetecting a deviation in the optical axis direction on the basis ofreflected light of the first light beam component focused at a firstnumerical aperture of the light beam focused by said objective lens;second focus detection means for detecting a deviation in the opticalaxis direction on the basis of reflected light of the second light beamcomponent focused at a second numerical aperture lower than the firstnumerical aperture of the light beam focused by said objective lens;addition means for adding a second focus error signal detected by saidsecond focus detection means to a first focus error signal detected bysaid first focus detection means; and drive control means for drivingsaid focusing actuator in accordance with an output from said additionmeans, wherein the first condensing lens condenses the first light beamcomponent without condensing the second light beam component by using aphase difference between the first and second light beam components, andthe second condensing lens condenses the first and second light beamcomponents, wherein said first focus detection means detects the firstfocus error signal on the basis of the reflected light of the firstlight beam component that forms a first optical spot with a first spotsize on an information recording surface of an optical disk and whereinsaid second focus detection means detects the second focus error signalon the basis of the reflected light of the second light beam componentthat forms a second optical spot with a second spot size larger than thefirst spot size on the information recording surface of the opticaldisk.
 2. An apparatus according to claim 1, further comprising drivecontrol means for controlling driving of said objective lens holder toadjust a focal point to an in-focus point of said second focus detectionmeans upon executing focus control with respect to the informationrecording surface of the optical disk.